1. Technical Field
The present invention relates to a coating liquid, a method for manufacturing an optical component, and a photographic optical system, in particular, an optical component used as a low refractive index material excellent in an antireflection effect.
2. Background Art
On a surface of an optical component constituting an optical instrument, to improve light transmittance, an antireflection film is formed.
When, in air, a low refractive index material of which refractive index nc isnc=√ng  (Formula 1)to the refractive index ng of a base material is coated at an optical film thickness of λ/4 to light having a wavelength λ, the refractive index theoretically becomes zero.
A general antireflection film is formed by vacuum depositing a material having the refractive index lower than that of a base material. As a low refractive index material, magnesium fluoride (MgF2) having nd=1.38 is in broad use. Here, nd is the refractive index to light having a wavelength of 587 nm.
When magnesium fluoride (nd=1.38) is disposed on an optical glass BK7 (nd=1.52) at an optical film thickness of λ/4, residual reflectance of 1.26% is generated.
In this case, to nullify the reflectance, the refractive index nc is necessary to benc=√nd(BK7)=√1.52=1.23  (Formula 2)
As an antireflection film of an optical element necessary to have a lower reflection effect, not a single layer but a multilayer film formed by alternately laminating a high refractive index film and a low refractive index film is used. Also in this case, a low refractive index material is important as the uppermost layer on an air side.
On the other hand, an attempt to make the refractive index smaller by forming a composite film with a low refractive index material is broadly conducted. When materials A (refractive index nA) and B (refractive index nB), which have different refractive indices, are mixed at a ratio of p:1−p, apparent refractive index n is represented byn=nA×p+nB×(1−p)=nB−p×(nB−nA)  (Formula 3).Herein, p represents the porosity.
It is suggested to be advantageous to form a porous film with a gas (usually, air) having the refractive index≈1 to obtain a low refractive index film. Herein, when the material A is air, nA≈1, accordingly, formula (3) becomesn=nB−p×(nB−1)  (Formula 4).
This is neither more nor less than the refractive index exhibited by a material having a bulk refractive index nB when the porosity thereof is p.
When magnesium fluoride (nd=1.38) is used as the low refractive index material to obtain a porous film having the apparent refractive index n=1.23, the porosity of about 40% is necessary.
As a method for preparing a porous film, not a dry process such as a vacuum deposition, but a wet process is effective. In the case of the wet process, after a coating material is dissolved or dispersed in a solvent, various coating methods can be used to deposit, and accordingly, there is an advantage that a porous film tends to be readily obtained.
On the other hand, examples of the methods where magnesium fluoride is prepared according to the wet process include methods illustrated below. U.S. Pat. No. 4,492,721 and M. Tada et al., J. Mater. Res., Vol. 14, No. 4, April 1999, p. 1610 to 1616 discuss a method where magnesium fluoride is prepared according to a thermal disproportional reaction. After a fluorine-containing magnesium compound or a magnesium fluorocarboxylate compound as a precursor is coated on a substrate, the thermal disproportional reaction is conducted to prepare magnesium fluoride. However, in both cases, the refractive index is around 1.39, that is, only a value of bulk magnesium fluoride is obtained. In addition, a deposition temperature thereof reaches 400° C. or 500° C.
U.S. Pat. No. 5,599,588 discusses a method for obtaining a rare earth and/or alkaline earth halide by at least hydrolyzing a halogenated alkoxide. However, since the halogenated alkoxide readily reacts with moisture in air to be instable, the deposition has to be conducted in an inert gas. Accordingly, an optical film has not been stably obtained.
As mentioned above, when a magnesium fluoride film is stably formed according to the disproportional reaction only by heating fluorine-containing organic magnesium compound, it is necessary to heat to 300° C. or more. However, there is fear of inducing deterioration of dimensional accuracy when a molded optical component is subjected to a high temperature, and, a further larger damage is inflicted depending on a material of an optical element. Accordingly, it is an issue to reduce a burning temperature.